1) Field of the Invention
The present invention relates to a developing device which creates an electrostatic latent image and an electrostatic recording device including the developing device. More particularly, the present invention relates to a developing device arranged in electrostatic recording devices such as electrophotographic printers and electrostatic recording-type printers and develops an electrostatic latent carrier which transfers an electrostatic latent image.
2) Description of the Related Art
FIG. 6 is a diagram partially illustrating a prior developing device. A photoconductive drum 1 creates a visible image to be transferred onto a paper. A magnetic developing roller 2 transfers a developer 6 onto the photoconductive drum 1 to develop an image onto the photoconductive drum 1.
A magnetic transfer roller 3 supplies the developer 6 stored in the developing device onto the developing roller 2. The doctor blade 4 regulates the amount of the developer 6 adhered to the magnetic developing roller 2 to prevent the developer 6 from being supplied excessively onto the magnetic developing roller 2.
The magnetic collection roller 5 collects the dispersed developer 6 into the developing device to reuse it.
Now let us explain briefly the structure of each of the magnetic developing roller 2 and the magnetic collection roller 5. Plural magnets are arranged in the internal portion radially and with respect to the rotational axis of each roller and covered with cylindrical sleeves. The internal magnets are fixed. Even if the roller is rotated, the internal magnets do not follow, but only the sleeves are rotated. Aluminum, for example, is used for the sleeves.
In order to print images to a sheet by the developing device with the above-mentioned structure, a latent image is first formed onto the photoconductive drum 1. Then the developer 6 stored in the developing device is supplied to the magnetic developing roller 2 via the transfer magnetic roller 3.
The doctor blade 4 regulates an excessive supply of the developer 6 to the magnetic developing roller 2. A magnetic brush (to be described later) with a predetermined height is grown on the magnetic developing roller 2.
Next, the developer (toner) 6 is supplied onto the photoconductive drum 1 on which a latent image is formed via the magnetic developing roller 2. Then a visible image is printed onto a sheet of paper by transferring the toner which is on the photoconductive drum 1.
In the brief explanation on the magnetic brush, a toner component (fine powder particles of colored resin) and magnetic components (fine magnetic carriers) coexist in the developer 6. The toner sticks onto the carrier by the electrostatic force, and the carrier on components ranged by magnetic force. The developer 6 transfers the toner with carrier. That is, the carrier attracts the toner components in plural fine particles. This state looks like the bristles planted on a brush and is called a magnetic brush.
However, in the above-mentioned developing device, the developer 6 is sustained only by the magnetic force of the magnetic developing roller 2. Hence the centrifugal force occurring as the magnetic developing roller 2 rotates or the wind pressure occurring as the photoconductive drum 1 moves may disperse the developer 6.
There is a place (magnetic force neutral area) at which the magnetic forces of the magnets arranged on the rollers 2 and 5 are balanced between the developing roller 2 and the magnetic collection roller 5, or area attraction does not occur due to the magnetic forces of the magnetic rollers 2 and 5. Such a magnetic force neutral area, as shown in FIG. 6, usually exists along the doctor blade 4 because of the relative positions at which components are arranged.
Therefore, the developer 6 dispersed first sticks on the surface of the doctor blade 4 arranged near to the developing area. It is considered that since the first dispersed developer 6 is triggered as an origin, the developer 6 dispersed accumulates continuously on the doctor blade 4.
With the developer 6 accumulated on the doctor blade 4, continuing the printing operation results in a further accumulation of the developer 6 or in contact or adhesion to the surface of the photoconductive drum 1 of the accumulated developer 6. In this case, the image created on the photoconductive drum 1 may be soiled so that the printed matter obtained by printing on a sheet may be soiled.
In order to continue good printing by preventing the above-mentioned problem, a maintenance man may manually remove the developer 6 accumulated on the doctor blade 4. However, increasing the speed of components including the photoconductive drum 1 and the magnetic developing roller 2 to execute a high-speed printing leads to an increase in the amount the developer 6 accumulated on the doctor blade 4 in a predetermined time. For that reason, it is necessary to shorten the maintenance term to clean the surface of the doctor blade 4.
Hence, the above-mentioned measure results in an increase in the personnel expenses because of frequent requests for maintenance, higher probability of soiled printed matter, and a device with lower reliability.
If some cause (vibration, an increased amount of accumulation) moves a mass of developer 6 accumulated on the doctor blade 4 onto the magnetic developing roller 2, the developer 6 may be locally thickened on the magnetic developing roller 4 so that an undesired amount of developer 6 is adhered to the surface of the photoconductive drum 1.
Hence, a large amount of the developer 6 accumulated on the member such as the doctor blade 4 existing near to the photoconductive drum 1 results in degradation in print quality.